In an electrical machine the uneven distribution of rotor mass around an axis of rotation causes a moment which gives the rotor a wobbling movement characteristic of vibration. This rotor mass unbalance excites elliptical whirling of the shaft of the rotor and limits the designing especially in large electrical machines.
A common solution for reducing vibrations is to use vibration dampers.
The drawback with the previous solutions for reducing vibrations originating from a rotor mass unbalance is a considerable effort and expense to manufacture, to assemble and to service the damping systems.